There has been great interest in the development of a zinc bromide battery as an energy storage device because of the potential for its simplicity of design, high theoretical voltage, and low cost of reactants. In such a battery the energy is stored by electrolyzing an aqueous zinc-bromide cell on charge to form zinc metal and bromine liquid. During charge bromine is evolved at the cathode and dissolved in the electrolyte while zinc metal is deposited on the anode. On discharge the two reactants are consumed to form zinc bromide.
An exemplary zinc bromide battery consists of a stack of flow frame assemblies wherein a carbon bipolar electrode is bonded into each frame. The flow channels in the frames direct electrolyte past the anode and cathode side of each electrode. One side of each electrode is usually a flat surface on which zinc is deposited and consumed while the other side of the electrode may comprise a carbon felt to support the bromine evolution and consumption reactions. A porous separator is maintained between the positive and negative sides of the adjacent electrodes to prevent bromine from diffusing from the positive electrolyte to the negative electrolyte, each of which is maintained in a separate flow system.
During charge of the battery a method of storage is required to remove generated bromine from the catholyte, to avoid increase in bromine concentration to levels of self-discharge and corrosion of cell components. Thus, bromine is stored as a complex with a quaternary ammonium bromide salt so that up to four bromine molecules can reversibly complex with the salt. The unbrominated quaternary salt is soluble in aqueous electrolyte while the polybromide complex is insoluble and separates out in a heavier oil-like phase. The organic complexing agent flows in a separate flow loop and is not pumped through the cell stack. Contact between the catholyte and complexing agent is accomplished by dispersing the complexing agent into droplets in a mixer external to the cell stack, thus increasing the area for bromine transfer between the two phases. An exemplary zinc bromide battery stack utilizing this system is disclosed in U.S. Pat. No. 4,162,351.
It is, however, desirable to scale up current zinc bromide technology to meet costs and performance requirements on a large scale, as for example for a power utility load leveling mission. To meet these requirements it is important to improve not only the efficiency of the design of the battery but also to improve the system lifetime, and in particular the lifetime of components which are subjected to particularly stressful conditions. One of these components is the bromine electrode, which includes the cathode side of the bipolar electrode. The flat surface of the cathode side of the bipolar plate may be used alone as the cathode, however, in order to provide an efficient battery this configuration requires high bromine concentration in the catholyte in order to reduce or avoid the effects of concentration polarization. Therefore, normally a high surface are material such as a carbon felt, in contact with the cathode surface of the bipolar electrode, is utilized to provide more surface area to support the Br.sub.2 /Br- reaction. The felt material is usually made of electrically conductive carbon and may also incorporate an insoluble stable catalyst to either enhance the Br.sub.2 /Br- reaction or to reduce activation polarization.
U.S. Pat. No. 4,235,695 discloses porous electrodes made of vitreous or glassy carbon forming a porous body with a coating containing an electrocatalytic agent. The porous vitreous carbon material is prepared by infusing polyurethane resin strands with a curable furan resin. The furan is polymerized then rapid heating carbonizes the structure to form a porous vitreous carbon. As disclosed, the carbon structures provide a density of between about 0.03 and 0.08 gm/cc. In the examples glassy carbon structures are disclosed having a porosity of about 70% and 45%.
U.S. Pat. No. 4,505,992 discloses porous gas distribution plate assemblies for fuel cells, which include an inner impervious region. The porous plates are disclosed as being. vitreous carbon needled-felt plates, or graphite plates having pore sizes in the range of 0.1 to 1.0 mm, 0.01 to 0.1 mm, and 0.001 to 0.001 mm, respectively, in size
Bipolar current collector-separators for electrochemical cells containing graphite and thermoplastic fluoropolymers are disclosed in U.S. Pat. Nos. 4,214,969 and 4,339,332. A bipolar plate substrate for electrochemical cell containing glassy carbon and a plastic such as polyvinylidene fluoride homopolymer is disclosed in U.S. Pat. No. 4,098,967. The above patents, however, are not directed to bipolar electrodes which meet the requirements of low cost, durability, and good electrical performance in a zinc bromide battery for large industrial application.
It is therefore an object of the present invention to provide novel porous electrodes which have improved stability to a stringent electrochemical cell environment, particularly to a zinc-bromide cell environment.
It is a further object of the present invention to provide novel porous electrodes which may be scaled up to large industrial applications while maintaining or improving physical strength, chemical stability to electrochemical cell environment, electrical performance, and component longevity.
It is another object of the present invention to provide a method for manufacturing improved porous electrodes for electrochemical cells.
It is yet another object of the present invention to provide an assembly of a pressure-molded porous composite and nonporous composite for use in electrochemical cells.
These and other objects will become apparent from the following description and appended claims.